for the tendency of group/team members to contribute less to a task
in a group
than they would individually
An example of this would be that in a tug of war when one person is pulling on the rope, they exert a much greater amount of energy than if they just one member of a larger team.
One main way to combat social loafing is to make each group/team member accountable for his/her actions (such as by having team members grade each other or analyze individual performance).
Interestingly, social loafing is a mostly Western phenomenon. A 1993 study by P.C. Earley showed that individuals in Israel and China performed BETTER in group situations, while their American counterparts performed worse (as expected). It was concluded that China and Israel are more collectivistic, thus people work together better than in the individualistic United States.
Node your homework!
Source: "Behavior In Organizations", 7th edition by Jerald Greenberg and Robert A. Baron